Traditional non-destructive testing of ferrous metals using ultrasound (UT) is typically accomplished with pulse sources that stimulate the plate under test.
The piezoelectric ultrasound transducer converts the pulse of electrical energy into an acoustic pressure wave (sound). The pressure wave is coupled the surface of the plate via the fluid in the reservoir, or by an equivalent acoustic coupler. Most of the energy (sound) is reflected from the front surface of the plate due to the acoustic impedance discontinuity. Some energy enters the plate travels through the plate and is reflected from the back surface of the plate back towards the front surface. Some of the reflected energy leaves the front of the plate and some is reflected towards the back surface of the plate. This reflection process continues. Energy is lost from the plate boundary surfaces for each reflection. Energy lost at the front of the plate travels back towards the ultrasound transducer where it is received and converted back to electrical energy. The electrical pulses received represent the two way acoustic front-to-back acoustic travel times. The thickness of the plate can be estimated by measuring the pulse to pulse travel time and knowing the velocity of sound in the plate under test. By searching for a reduction in the plate thickness, one can locate plate corrosion or pitting.
The industry has long been aware of the need to inspect the walls of fluid reservoirs, particularly when those walls are metal. In this application, the term “walls” refers to both bottom and side walls of a structure or tank. Special problems are engendered when the fluid maintained in the reservoir is combustible.
One solution to the problem is the technology described in Silverman U.S. Pat. No. 5,205,174. This patent describes a robotic vehicle which can travel into a partly filled fluid reservoir and traverse the tank for inspection. The '174 patent describes both optical and ultrasonic inspection technologies.
The pulse echo technique works well on pristine plate. High pulse amplitudes (250+ volts) are required to obtain reasonable signal to noise ratios when debris or rough surfaces (corrosion) are present on the plate. The transducer is typically stimulated or “pinged” with a narrow pulse with duration in the 100 nanosecond range. The mechanical properties of the ultrasound transducer control the transmitted acoustic pulse. The measurement time or thickness resolution is primarily controlled by the transducer's acoustic bandwidth. If the transducer rings for an extended period of time, this can mask small returns close to the front surface as would be seen from the front surface. Additionally, the front surface energy is typically large with respect to the desired wall reflections and is normally clipped in the receiving electronics.
A particular problem in tanks storing combustible fluids is that the inspection system must first travel through a region devoid of the fluid before traveling into the fluid containing region in order to reach the wall to be inspected. Traversing the fluid-free regions is a problem because when the reservoir stores combustible fluid, the fluid-free region typically contains combustible vapors.
Standards bodies have devised standards for instruments which work or travel through regions containing combustible vapors. Limitations on the voltage and current levels found on apparatus in the combustible region allows that apparatus to be considered “intrinsically safe”.
On the other hand, limiting the inspection system to voltage and current levels which are below the thresholds established in these standards presents problems in conducting an effective inspection.